Copper mercaptides as sulfur dioxide indicators

ABSTRACT

Organophosphine copper(I) mercaptide complexes are useful as convenient and semiquantitative visual sulfur dioxide gas indicators. The air-stable complexes form 1:1 adducts in the presence of low concentrations of sulfur dioxide gas, with an associated color change from nearly colorless to yellow-orange. The mercaptides are made by mixing stoichiometric amounts of the appropriate copper(I) mercaptide and phosphine in an inert organic solvent.

BACKGROUND OF THE INVENTION

The invention described herein relates to organophosphine, arsine, orstibine copper(I) mercaptide complexes of the general formulaCu(MR₃)_(n) (SR') where M=P, As, or Sb and R and R' are organo groupsand their use as SO₂ indicators and scavengers.

Sulfur dioxide is a major pollutant in a great variety of industrialprocesses, and the monitoring of this gas is necessary forenvironmental, health, and engineering considerations. At this time,there are instrumental methods of direct SO₂ gas analysis but virtuallyno generally recognized dry chemical indicators.

SUMMARY OF THE INVENTION

We have found that organophosphine copper(I) mercaptide complexesreadily undergo adduct formation with SO₂ in low concentration, with anassociated color change from essentially colorless to yellow or brightorange. The arsines and stibines react similarly. Accordingly, in itsbroad scope, our invention encompasses a method of detecting SO₂ in anambient atmosphere which comprises (a) contacting the atmosphere with acomplex of the general formula Cu(MR₃)_(n) (SR') where M is P, As, orSb, n is 1 to 3, and R and R' represent an unsubstituted alkyl,cycloalkyl, aralkyl, or aryl group or such a group containing inertsubstituents, and (b) monitoring any color change produced in thecomplex as the result of formation of an SO₂ adduct. The mercaptides aremade by mixing stoichiometric amounts of the appropriate copper(I)mercaptide and a phosphine, arsine, or stibine in an inert organicsolvent. The resultant complexes serve as indicators for SO₂concentrations at least as low as 100 ppm when in the solid state,solution, or suspended on a filter paper support. The SO₂ is boundreversibly, and the adducts possess low dissociation pressures (˜1 torr)at ambient temperature and high dissociation pressures at 100° C. Thus,the copper mercaptides disclosed herein can also be used as scavengersfor SO₂ in process gases by merely heating them to release the boundSO₂.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Lewis acid character of SO₂ toward a wide variety of main groupbases is well known. However, only a few compounds have been reported inwhich SO₂ bonds to coordinated ligands. We have now found thatorganophosphine copper(I) complexes of the general formula Cu(PR₃)_(n)(SR') readily react with SO₂ at room temperature to form bright orangeto yellow adducts. Since the Cu(PR₃)_(n) (SR') complexes are nearlycolorless, the formation of the brightly colored adducts provides aready indication of the presence of SO₂.

As a consequence, these complexes are easily adaptable to a visualdetection method for low concentrations of SO₂ which is specific,inexpensive, semiquantitative, rapid, and simple. Furthermore, they canbe regenerated after use merely by allowing them to stand in SO₂ -freeair or by mild heating at ˜100° C., whereupon the bound SO₂ is released.However, removal of SO₂ from the adducts can best be effected bydissolving them in organic solvents and slowly removing solvent invacuo. In a preferred embodiment, a concentrated solution of the complexis evaporated on a strip of filter paper which then serves as thedetector. Exposure of the filter paper to air containing SO₂ produces acolor change almost immediately, with the variation in color beingdependent on the amount of SO₂ and the length of exposure.

Alternatively, the complexes may be used as wet rather than dry SO₂detectors in that the air or other gas containing SO₂ may be bubbledthrough a solution of the complex. The solution then takes on thecharacteristic orange or yellow color as the adduct is formed.

As used in this specification, in the general formula Cu(PR₃)_(n) (SR')n is 1-3, and R and R' represent an unsubstituted alkyl, cycloalkyl,aralkyl, or aryl group or such a group containing inert substituents.For the types of groups which may generally be represented by R and R',see U.S. Pat. No. 3,336,392 and the discussion therein with respect toformula III of that patent which is incorporated herein by reference.Specifically, R and R' include but are not limited to Me, Et, Ph, Bz,Cy, and C₆ F₅ where Me is methyl, Et is ethyl, Ph is phenyl, Bz isbenzyl, and Cy is cyclohexyl. It should be noted that while thephosphine complexes are preferred, SO₂ adducts also readily form withthe arsine and stibine complexes.

The nature of the organic substituents on both the phosphine andmercaptide ligands influences the stability of the SO₂ complexes. Itappears that adducts with ligands of high basicity are more stable thanthose with less basic ligands, which is not surprising since the SO₂ isbehaving as a Lewis acid toward the XR⁻ group.

The Cu(PR₃)_(n) (SR') complexes are easily prepared by dissolving theknown polymeric mercaptides, CuSR', in chloroform or another appropriateorganic solvent containing n (in some cases n+1) moles of phosphine(PR₃) per mole of CuSR'. The CuSR' is readily prepared by the method ofDucan, Ott, and Reid, Ind. & Eng. Chem. 23, 3181 (1931). In someinstances it is difficult to crystallize the Cu(PR₃)_(n) (SR') speciesfrom solution due to oil formation. However, addition of excess SO₂ gasto the solution will induce crystallization of yellow to orange SO₂adducts, Cu(PR₃)_(n) (SR')(SO₂), which, e.g., on controlled heating invacuo, will release SO₂ and yield solid Cu(PR₃)_(n) (SR'). To the extentthat certain of the phosphines may be oxygen sensitive, the complexes ofthese phosphines are readily prepared in an inert atmosphere. If thecomplexes are soluble in the organic solvent used, addition of astraight chain alkane such as n-heptane or n-hexane facilitates theirremoval from the reaction mixture.

EXAMPLE I

A mixture of PBz₃ and Cu(SPh) in a 3:1 molar ratio was stirred in CHCl₃(minimum quantity to dissolve PBz₃) under nitrogen until the mercaptidedissolved (10-30 minutes). The resulting solution was treated with asmall amount of decolorizing carbon and filtered. Excess n-heptane(n-hexane can also be used) was added until precipitation of the nearlywhite Cu(PBz₃)₂ (SPh) was complete. The flocculent solid was collectedby filtration, washed with hexane, and dried in vacuo at 50° C.Elemental analysis (Theory for C₄₈ H₄₇ P₂ SCu: %C, 73.8; %H, 6.1. Found:%C, 72.4; %H, 5.9) indicated the composition of the solid to be asstated. Yield was essentially quantitative.

EXAMPLE II

When prepared in accordance with Example I, certain of the Cu(PR₃)_(n)(SR') complexes are obtained as oils. To convert such oils to solids,the complex is dissolved in diethyl ether and SO₂ is slowly bubbledthrough the solution. A bright orange color develops immediately andcrystals of the 1:1 SO₂ adduct begin to deposit. The product iscollected on a filter, washed with SO₂ -saturated ether, and dried in astream of SO₂. The yields of Cu(PR₃)_(n) (SR')(SO₂) depend on solubilityand other factors but are generally 60-90%. Complexes which are readilyobtained in this way include those for n+3, R₃ +MePh₂ (R'+Ph) and n=2,R₃ =Ph₃ (R'=Me, Et, C₆ F₅), R₃ =MePh₂ (R'=Me), R₃ =Bz₃ (R'=Et, Ph), andR₃ =Cy₃ (R'=Me, Ph, Cy). The SO₂ bonds to the mercaptide sulfur in theseadducts, as demonstrated by the x-ray crystal structure of Cu(PPh₂ Me)₃(SPh)(SO₂). See Eller and Kubas, J. Amer. Chem. Soc. 99, 4346 (June 22,1977), which is incorporated herein by reference.

Generally solid SO₂ -free complexes can be readily obtained by heatingthe adducts in vacuo to 100°-150° C. Slight loss of phosphine may occur,especially at higher temperatures. This, however, does not appear toaffect the ability of the complex to form SO₂ adducts. The SO₂ is alsoreleased slowly by dissolving the adducts in organic solvents andremoving solvent in vacuo at ambient temperature.

EXAMPLE III

The solid mercaptides and their solutions in organic solventsimmediately turn yellow to orange when exposed to gaseous SO₂, even inlow concentrations. A concentrated solution of Cu(PCy₃)₂ (SPh) in CHCl₃was evaporated on a strip of filter paper. Exposure of the paper toair-SO₂ mixtures containing as little as 100 ppm SO₂ produced asubstantial yellow coloration in the paper after about 3 minutes. Adetectable color change occurred almost immediately and was more intenseupon exposure of the paper to higher concentrations (>1000 ppm), thusindicating that semi-quantitative measurements are possible. Removal ofthe test strips from the SO₂ -containing environment to the normallaboratory atmosphere resulted in a slow fading of color. After 2-3days, the strips were once again nearly colorless and reusable since anew exposure to SO₂ gave substantially the same color change as before.High concentrations of nitrogen oxides (NO and/or NO₂) darkened the teststrips, but did not give a yellow or orange coloration. Cu(PPh₂ Me)₃(SPh)-impregnated filter paper gave similar results. Althrough thereappears to be some variation in the lower limit of sensitivity of thevarious complexes, this variation may be used for quantitativemeasurements in that a test paper can be impregnated with calibratedbands of complexes with different continuous ranges of sensitivity.

The foregoing examples are not intended in any way to limit the scope ofthe invention but rather are presented for the purpose of meeting theenablement and best most requirements of 35 U.S.C. 112. The scope of theinvention is as set forth in the Summary of the Invention and the broadclaims appended hereto.

What we claim is:
 1. A method of detecting SO₂ in an ambient atmospherewhich comprises (a) contacting said atmosphere with a complex of thegeneral formula Cu(MR₃)_(n) (SR') wherein M is P, As, or Sb, n is 1 to3, and R and R' represent an unsubstituted alkyl, cycloalkyl, aralkyl,or aryl group, and (b) monitoring any color change produced in thecomplex as the result of the formation of an SO₂ adduct.
 2. The methodof claim 1 wherein M is P.
 3. The method of claim 2 wherein said complexis in solution and said atmosphere is passed through said solution. 4.The method of claim 2 wherein said complex is a solid disposed on asuitable substrate.
 5. The method of claim 4 wherein a plurality of saidcomplexes having different sensitivities are disposed in a desiredsequence on said substrate.
 6. The method of claim 3 or 4 wherein R andR' are methyl, ethyl, phenyl, cyclohexyl, or C₆ F₅.
 7. A detector forSO₂ in an ambient atmosphere which comprises Cu(PR₃)_(n) (SR') disposedon a suitable substrate, wherein n is 1 to 3, and R and R' represent anunsubstituted alkyl, cyclohexyl, aralkyl, or aryl group.